Need subsonic filter - Page 8 - Home Theater Forum and Systems - HomeTheaterShack.com

Old 08-31-09, 07:31 PM
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SturmMD

Join Date: Dec 2008
Location: Minneapolis, Minnesota
Posts: 165
IC for generating negative voltage rail

Using an IC such as this and shown in the circuit example earlier in the thread is a viable way to generate the negative supply voltage for the circuits shown. It does require a little bit of care and I will describe some of the considerations that must be made.

First, consider the range of voltages the IC can handle. Some variants of the *7660 charge pump can handle up to 10V, others 12V.

Second, the IC has limited output capability. This is expressed by the output resistance. The datasheet for this part specifies 60 ohms. This means that if it is given 10V it ideally generates -10V from that. If a total of 10mA are being drawn from it this is reduced by (60ohms * 10mA) = 0.6V so the output is actually only -9.4V.

Because of this fact it is important to consider the amount of power the circuits are using from the IC. For opamps, the power requirements have static and dynamic components. Each opamp draw a relatively constant amount of power while on, this is specified as a current, the quiescent current. Multiply this number by how many opamps you have - so for example a TL072 has 2 opamps that each draw 2mA, or a NE5532 uses 5mA per opamp.
The dynamic component varies with the output signals and the load they drive. This is usually small except when the output is driving a 600ohm load. For example, a +16dBu signal (4.88Vrms) driving 600ohms places an additional 8mA demand on the power supply.

Sum together all the different components of your circuit to determine the average and peak current demands and observe how that affects your negative rail voltage under those two conditions

Uneven positive and negative supply voltages are ok, just remember the circuit is limited by whichever supply has a lower voltage (absolute value)

The final factor is that the output voltage swing of opamps is limited and does not extend all the way to the supply voltages. The output generally ranges to within 2 to 3V of the supply. So extending the example above (+10V supply, -9.4V when 10mA load). This means that you should not expect the circuit to be able to output more that a 6.4V peak (9.4V - 3V) without distortion. This corresponds to about +15dBu
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Old 09-01-09, 02:56 PM
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Hugo

Join Date: Jan 2009
Location: SATX
Posts: 96
Re: Need subsonic filter

Quote:
SturmMD wrote: View Post
I have actually been working on a SSF project that includes a 3 digit LCD and microcontroller to have 0.1Hz resolution adjustibility from 10-25Hz. More on this in the next 2-4 weeks. When I get some prototype PCBs made for this I'll see if I have some room for something like this thread.
How is your kit coming along? Any word on availability?
Old 09-01-09, 04:30 PM
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SturmMD

Join Date: Dec 2008
Location: Minneapolis, Minnesota
Posts: 165
Re: Need subsonic filter

Quote:
How is your kit coming along? Any word on availability?
I have designed the pcbs and am figuring out software to combine several different pcbs on the same board to lower manufacturing costs for the first prototypes. When those come in I have to assemble them and work on the microcontroller software.
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Old 09-01-09, 04:46 PM
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Hugo

Join Date: Jan 2009
Location: SATX
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Re: Need subsonic filter

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SturmMD wrote: View Post
I have designed the pcbs and am figuring out software to combine several different pcbs on the same board to lower manufacturing costs for the first prototypes. When those come in I have to assemble them and work on the microcontroller software.
That's cool, divide and conquer. Will you be incorporating the variable/fixed volume gain and the balanced output in your design? This would be exactly what I need. The sub out on my sony receiver isn't enough to drive the ep2500. I'm using a samson bump box. I'll be stepping up and getting a dsp1124. I'll still learning, but I believe I will still need a high pass filter. My box is tuned to 28hz, but I watch a bunch of movies through the system as well. I don't want to over extend my avalanches, so I just don't turn it up as loud. =( Any who, good luck and keep us posted. Thanks.
Old 09-01-09, 04:52 PM
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SturmMD

Join Date: Dec 2008
Location: Minneapolis, Minnesota
Posts: 165
Re: Need subsonic filter

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Will you be incorporating the variable/fixed volume gain and the balanced output in your design?
I'm looking at having 8 fixed gain settings; no knob, just the buttons that are used for changing SSF frequency too. And, yes, balanced output.
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Old 10-14-09, 08:34 AM
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Gwyneth

Join Date: Oct 2007
Posts: 4
Re: Need subsonic filter

Thanx guys, this is just what we need for our new sonosub. We're going with a+/-12v supply as we will use the Parts Express wall wart ac supply. Not having a big parts bin, we found most of the parts we needed at Tayda Electronics. Their prices seem reasonable to us. We will also recycle some parts (6800uF caps for in the power supply) from our spare bits from an Apexjr / Cerwin Vega sub amp. Thanx Again.
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Old 12-25-09, 10:41 AM Thread Starter
buzz

Join Date: Oct 2006
Location: singapore
Posts: 49
Re: Need subsonic filter

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collo wrote: View Post
OK, so that would make the simplified circuit...

I did not have time for the past 6 months to implement the active filter and test. I only got the time now to test it. I have built the circuit shown above 6 months ago. It should work right ? I notice that many changes have been made to the circuit and newer versions are available. When i power up the amplifier and connect to a speaker (this circuit should pass frequencies above the cutoff frequency to 20Khz right ?), the volume is very low. Is there a major problem with the circuit ? I use unregulated +12V and -12V to power the high pass filter. Anyone can suggest what i should do to troubleshoot. Thanks.

Last edited by buzztiger; 12-25-09 at 10:47 AM.
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Old 12-25-09, 12:53 PM
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SturmMD

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Re: Need subsonic filter

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buzztiger wrote: View Post
When i power up the amplifier and connect to a speaker (this circuit should pass frequencies above the cutoff frequency to 20Khz right ?), the volume is very low. Is there a major problem with the circuit ? I use unregulated +12V and -12V to power the high pass filter. Anyone can suggest what i should do to troubleshoot. Thanks.
Is the volume much different from when you aren't using the circuit - because it shouldn't be drastically different. If you want to try adding gain you just need to add two resistors to the first opamp's circuit as shown in the newer version of the circuit. You have a large enough supply voltage that you should be fine putting a lot of gain. Maybe start out with a gain of 3 (about 10dB) by using the same 2R and R resistors used in the filter section also for the gain setting.
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Old 12-25-09, 11:42 PM Thread Starter
buzz

Join Date: Oct 2006
Location: singapore
Posts: 49
Re: Need subsonic filter

I forgot to mention one more thing. I am using a 10K linear pot. Would this also contribute to the problem ?

The volume was way louder without the active filter. Now with the filter circuit i need to put my ear on the speaker to hear anything with the linear pot turned to 0 ohm.
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Old 12-26-09, 09:16 AM
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SturmMD

Join Date: Dec 2008
Location: Minneapolis, Minnesota
Posts: 165
Re: Need subsonic filter

Are you using exactly the circuit you posted above? There is no spot for a potentiometer in it?
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